Soft Bounce
What is Soft Bounce?
A temporary delivery failure (`4xx` or storage-related `552` SMTP code) where the recipient's email address is valid, but the message was deferred due to a full mailbox (`FULL_INBOX`), temporary server throttling, greylisting, or message size limits.
Why Soft Bounce Matters in Production
Many senders ignore soft bounces assuming they will resolve automatically. In reality, an abandoned consumer mailbox that has hit its 15 GB Google/iCloud storage cap (`452 4.2.2 over quota`) will bounce every single campaign you send—and Gmail eventually converts persistent over-quota soft bounces into permanent hard bounces.
Engineering & Sender Best Practices
- • Isolate `status: "FULL_INBOX"` during pre-campaign verification and pause sends to those contacts for 30 days.
- • Automatically suppress any subscriber that soft-bounces across 3 to 5 consecutive campaigns.
- • Keep HTML email payloads under 100 KB and attachments hosted via links to avoid message-size soft bounces.
- • Pace high-volume sends to corporate domains to avoid `421` concurrent-connection rate-limit deferrals.
Pre-Empting Holiday E-Commerce Soft Bounces from Over-Quota (`FULL_INBOX`) Mailboxes
During Q4 holiday campaigns, a retail brand noticed 3,400 subscribers repeatedly soft-bouncing with `452 4.2.2 The email account that you tried to reach is over quota` on Gmail and iCloud. Because their ESP retried soft bounces for 72 hours per campaign, their sending queue backed up and Gmail downgraded their domain reputation. Pre-verifying with MailVeri flagged `status: "FULL_INBOX"` before launch so those 3,400 contacts could be paused for 30 days.
Technical Implementation & Wire Example
{
"email": "[email protected]",
"status": "FULL_INBOX",
"smtp_code": "452 4.2.2",
"diagnostic": "Mailbox full / storage quota exceeded",
"credits_used": 1,
"recommended_action": "pause_30_days"
}MailVeri separates FULL_INBOX from generic VALID/INVALID statuses so you don't waste sends on full mailboxes.
SMTP Reply Codes & Bounce Classification Reference
RFC 5321 & Enhanced RFC 3463 Status Codes| SMTP Code | Bounce Classification | Server Meaning | MailVeri Status & Action |
|---|---|---|---|
| 250 2.1.5 | Deliverable (2xx) | Recipient RCPT TO accepted by destination MTA | VALID (or CATCH_ALL if wildcard server) |
| 452 4.2.2 | Soft Bounce (Storage) | Recipient mailbox has exceeded storage quota | FULL_INBOX — pause sends for 30 days |
| 421 / 450 4.7.1 | Soft Bounce (Transient) | Temporary greylisting or concurrent connection deferral | UNKNOWN (0 credits billed) |
| 550 5.1.1 | Hard Bounce (5xx) | User unknown / mailbox does not exist on server | INVALID — suppress permanently |
| 550 5.7.1 | Policy / Reputation Block | Rejected due to sender reputation, RBL, or DMARC failure | Check sender domain DNSBL & authentication |
Full Inbox (`FULL_INBOX`) & Soft Bounces: The Silent Deliverability Killer →
Why over-quota mailboxes ruin seasonal e-commerce and newsletter deliverability—and how MailVeri flags FULL_INBOX pre-send.
Free Bounces & SMTP Signals Tools & Workflows
Frequently Asked Questions About Soft Bounce
Why do Google and iCloud accounts experience so many `452 4.2.2` full-inbox soft bounces?
Free Google accounts share a 15 GB storage cap across Gmail, Google Photos, and Google Drive (and free iCloud accounts share 5 GB). Once a consumer's cloud photo backup fills their quota, their Gmail/iCloud address immediately rejects all incoming emails with a 452 over-quota error.
Does a full-inbox soft bounce eventually turn into a hard bounce?
Yes. If a mailbox remains over quota for months without the owner clearing space or upgrading storage, ISPs eventually escalate rejections or deactivate the dormant account into a permanent `550` hard bounce.
How is a `421` soft bounce different from a `452` soft bounce?
`452 4.2.2` is a recipient-specific storage issue (`FULL_INBOX`), meaning you should pause mailing that specific user. `421 4.7.0` is a server-level rate limit or temporary throttling deferral, meaning you should slow down your sending concurrency to that domain.
Closely Related Terms
Explore All Email Verification Terms (A–Z)
Glossary Index →Ready to start verifying emails?
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